It is known that corresponding line-connected supply grids can be available for the transmission of electrical energy. Depending on the electrical power to be transmitted, these supply grids have a rated voltage of, for example, 380 kV, 110 kV or else 10 kV, wherein a mains frequency of 50 or 60 Hz can be used. A supply grid for the supply of power to stationary consumers can have a three-phase design. In this case, a system with three supply lines is made available in which, in the balanced state, current and voltage can be equal in terms of magnitude with a phase shift of in each case 120° with respect to one another.
Energy supply systems for mobile consumers such as, for example, railways or tram systems can have a single-phase design. The supply of power takes place via a single supply line, wherein the return line is then provided via the metallic rail. In the case of trolleybuses, owing to the rail which is not present and therefore cannot be used as return conductor, two or more supply lines can be provided. In general, the mains frequency in such applications is 16⅔ hertz, for example, in Europe, and in some cases such as tram systems, in individual cases DC voltage is also used.
For the transformation of the AC supply voltage from 10 kV to 15 kV, mobile transformers can be provided which can then be integrated, for example, in the underfloor region of a passenger train.
These transformers only have a very limited amount of room available, in particular in respect of height, owing to the underfloor arrangement and are usually in the form of oil-type transformers. In this case, the oil first acts as coolant for dissipating the lost heat produced during operation and also as insulation, by means of which relatively small insulation gaps and therefore a compact design can be realized.
One disadvantage with this configuration, however, is that such a transformer can usually only be arranged vertically for mechanical reasons, but this is in opposition to the flat space available in the underfloor region. In addition, for safety reasons, oil should wherever possible be avoided as combustible medium in a vehicle. In this case, the cooling effect of the oil is lost.